Passively Q-switched microchip lasers have many advantages: they allow the realization of low-cost, compact and robust laser sources. Because of the short cavity length, they are capable of generating pulses with high peak power, which are particularly convenient for generation of harmonics (e.g. ultraviolet lasers). High peak power is of primary interest for many applications such as marking, excitation of fluorescence, ionization of solids such as matrix-assisted laser desorption ionization (MALDI) for proteomics, long distance time-of-flight ranging, generation of supercontinuum, and others.
As is known from prior art, the output performance parameters (pulse duration, energy, and repetition rate) of passively Q-switched lasers based on saturable absorbers depend primarily on the design of the microchip laser cavity. These include the optical path length through the different materials, magnitude of gain in the active medium, saturable and non-saturable losses, output coupling, length of the cavity. According to basic laser equations, the main trends are:                the repetition rate depends linearly on the ratio of the pump power intensity to the laser threshold intensity;        the pulse energy is directly proportional to the pumped volume, and to the total losses; and        the pulse duration is proportional to the cavity length, and inversely to the losses.        
Consequently, for a given design of the microchip (i.e. for a fixed laser threshold), the pulse duration is approximately constant, while the pulse energy does not vary with pump power if the focusing of the pump is unchanged. Thus, repetition rate is the sole performance parameter that can be controlled through pump power only. To achieve control of the peak power, a more complex design of laser is involved.
Changing the pulse energy requires an adjustment of the pump beam focus by translating the focusing optics to adapt its magnification. However, positioning tolerances for such optics are generally very tight (a few microns) so that the accurate control of the pulse parameters requires a high precision mechanical mounting scheme which is not very practical and considerably expensive.
Another alternative would be to use an output-coupling mirror with a reflection coefficient that varies across the surface of the microchip, so that the cavity losses could be adjusted by translating the microlaser laterally. However, this solution is also not very practical and technically very difficult to implement given the typical lateral dimensions of a microlaser. It would also be very expensive since it would require that every microchip be coated independently, thereby losing the benefit of mass processing, which is a major advantage that the microchip concept has introduced.
For actively Q-switched lasers the situation is obviously completely different, as they possess a larger number degrees of freedom. Many ways of controlling the pulse energy and the repetition rate have been disclosed in prior art, based mainly on the proper control of the Q-switch element. However, passively Q-switched lasers have some key advantages over actively Q-switched lasers, such as size, cost, peak power, etc. which make them unique solutions for some applications.
In conclusion, to the best of our knowledge, no simple means has been disclosed for controlling the output pulse parameters of passively Q-switched lasers.
However, the ability to independently control the pulse energy or the peak power density is a key factor for some applications. Here is a non-exhaustive list of examples:                in MALDI, the peak power density has to be larger than the ionization threshold; varying the laser spot size on the surface of the sample allows the number of ionized molecules to be changed, so that molecules with different responsivity can be analyzed;        in time-of-flight ranging, the peak power impacts on the range, while the spot size determines the lateral resolution for scanning systems,        in machining applications, the spot size determines the resolution, while the peak power impacts on the machining speed.        
According to the basic trends detailed above, the laser output parameters of a passively Q-switched microchip laser can be controlled by varying the lengths of gain media and saturable absorber media that are present within the laser cavity.